Whenever the film cuts to the control panel's list of names, several are listed in "First name, Last name" format instead of the standard "Last name, First name" format used for all of the close-up shots. Additionally, some are misspelled, such as "Stve".
The building supposedly became a giant defacto Faraday cage, but they are able to pick up a local radio station from inside the building. But nothing supports that the metal around the building is also acting as a Faraday cage. In many outside shots the of building you can see a tower on the roof with 3 Sector antennas positioned around it, this is likely a cell tower and due to the remote location likely the only one providing service to the area. With the level of expertise shown by the perpetrators it would be simple to disable this tower during the lockdown. It is also easily more probable a cell jammer or jammers could have been activated in or near the building.
When Dany and Roberto are trapped above the moving elevator, it shows Barry going to the top floor. While this is convenient for the scene, it doesn't make sense for Barry to be heading to an empty floor immediately following orders to kill as many people as possible. Furthermore, in the very next scene he is suddenly on the first floor.
Near the end when Mike is the only survivor, the shutters (at least in the lobby) are seen opening from the inside of the building. As the guards lead Mike away from the building, all of the first floor shutters are closed.
When Michael Rooker's Character is killed, a green screen patch was used on his head to CGI a dent in his skull. In the final wide shot of the character, the Green Patch with no CGI enhancements can be visible in frame.